Use MySQL error names instead of error codes

Currently, Executor::MySQL.pm contains a table of hard-coded error codes and their mapping to RQG exit codes. Instead, error names should be used throughout, since error codes change sometimes, making a particular error unclassifiable.

Error names are available from include/*.h and sql/share/*.txt . In addition, each error name should be systematically classified and mapped to a RQG status code, in contrast to the current scheme where errors are only defined when they are encountered.